!! is just ! (the boolean negation operator) written twice. It will negate the argument, then negate the negation. It's useful because you can use it to get a boolean from any value. The first ! will convert the argument to a boolean, e.g. true if it's nil or false, and false otherwise. The second will negate that again so that you get the boolean value of the argument, false for nil or false, true for just about everything else.
In Ruby you can use any value in an if statement, e.g. if current_user will execute if the current user is not nil. Most of the time this is great because it saves us typing explicit tests (like if !current_user.nil?, which is at least six characters longer). But sometimes it might be really confusing if you return an object when the method implies that it returns a boolean. Methods whose name ends with ? should return truthy or falsy values, i.e. they return something that will evaluate to true or false. However, it can get really messy if signed_in? returned a user object. For example if you're trying to debug why some code that uses signed_in? doesn't work you will probably get really confused when a user object turns up where you expected true or false. In that situation it's useful to add !! before the return since that guaranteed that the truthy or falsy value will be returned as either true or false.
